The present invention relates to a protective cover for a solar array or an array of solar cells, and, more particularly, to an optically self-aligning, output-increasing, ambient-protecting cover for an array of solar cells.
Various types of solar arrays for converting radiant energy, such as sunlight, into electricity are known. One type of array which is of particular current interest comprises a plurality of spaced members, typically spherical cells, supported by a flexible conductive sandwich which includes first and second foils separated by an insulative lamina. Each sphere is a semiconductor, for example silicon having a P-type interior and an N-type exterior or skin.
The spheres are loaded onto a foil matrix to form a cell sandwich. The foil matrix comprises the first foil, which is a thin, flexible metallic foil, typically aluminum, with a plurality of spaced cell-retaining apertures formed through the foil, for example by an emboss-then-etch or stamping process. The apertures typically define a regular geometrical pattern. Preferably, the pattern is a hexagon which permits maximum packing of the apertures (and, hence, of the spheres or cells) in the array. Loading the spheres involves placing an excess thereof onto a top surface of the matrix and, by the use of negative pressure and mechanical doctoring techniques, effecting the presence of one sphere in each aperture. Thereafter, heat and pressure are applied to the cell sandwich to move the spheres partially into and through the apertures. This movement effects the reaction of the aluminum foil with the very thin native silicon oxide layer on the spheres to locally remove the silicon oxide so that the abutting aluminum mechanically bonds directly to, and forms an electrical contact with, the N-type exterior of the spheres.
The affixing of the spheres to the foil matrix results in an upper light-gathering portion of each sphere protruding above a top surface of the first foil and a lower portion of each sphere protruding below a lower surface of the first foil. The lower portion of the each sphere is subjected to selective etching which removes the N-type exterior therefrom below the lower surface of the first foil to thereby expose the P-type interior of each sphere below such lower foil surface.
The lower foil surface and the exposed P-type lower sphere portions are then coated with a flexible, electrically insulative lamina, typically a polyimide, following which the second foil, preferably a flexible aluminum or other metallic foil, is electrically connected to the P-type interiors of the spheres. This latter electrical connection may be effected by selectively removing a portion of the polyimide to permit access to the P-type interiors of the spheres and then mechanically and electrically connecting the second foil to contacts engaging the P-type interiors and residing in holes formed in the lamina.
The foregoing and functionally similar solar arrays and techniques for producing them are disclosed in commonly assigned U.S. Pat. Nos. 5,192,400; 5,091,319; and 5,086,003; and in prior art cited therein.
The above-described solar array comprises a plurality of miniature solar cells--the spaced silicon spheres or other semiconductive members--connected in electrical parallel via the first and second foils. The foils, therefore, are connectable to a utilization device or circuit for electrical energization thereof when the array is exposed to radiant energy. The array is flexible and may be formed into various non-planar configurations, either free-standing or to conform to an irregular underlying surface.
While solar arrays constructed as set forth above are mechanically robust, protecting the arrays from the deleterious effects of the environment and ambient conditions is generally thought to be prudent. For example, water in the form of rain or other precipitation, in prolonged direct contact with the spheres or the foils can give rise to mechanical and/or electrical degradation of the array. Pollutants may also deleteriously affect the array, such as by attacking or preventing the functioning of the various elements thereof.
For the foregoing and other reasons, it is typical to cover, encapsulate or otherwise house solar arrays to protect them against ambient-caused degradation. Such protective measures have often taken the form of a rigid "picture frame" having a transparent cover which surrounds the solar array to isolate it from the ambient. The cover, of course, permits sunlight and other radiation to reach the cells or spheres where it is converted to electricity. Such picture frame covers are not flexible and limit the range of uses to which the flexible arrays may be put.
The upper portion of each sphere--an N-type silicon hemisphere--functions as a spherical lens. That is, this upper portion gathers light incident on the sphere and directs this light onto the sphere's P-N junction. These spherical lenses are able to direct to the junction only that light which is directly incident on the spheres or cells. Some of the light which is incident on the top surface of the first foil between the cells--that is, light which "misses" the spheres--is, in effect, "wasted" and is not effective to produce electricity, because it does not reach the P-N junction of the spheres, and is, instead, either absorbed by the top surface of the first foil or is back-reflected to the ambient.
An object of the present invention is the provision of a cover for solar arrays which encapsulates and protects the arrays from the ambient and which effects the direction onto the cells of a significant amount of the otherwise "wasted" light incident on the solar arrays. Another object of the present invention is the provision of a flexible cover having the foregoing characteristics which permits the forming of flexible solar arrays to assume non-planar configurations. Yet another object of the present invention is the provision of methods for producing the aforenoted cover.